1. Field of the Invention:
This invention relates to a well tool, such as a packer, used to seal the annulus between inner and outer conduit strings and, more particularly, to a well packer employing nonresilient and nonenergizing packing and sealing elements for maintaining sealing integrity under extreme temperature and pressure conditions, including the presence of saturated steam.
2. Description of the Prior Art:
Conventional subterranean well tools, such packers and bridge plugs, commonly employ annular elastomeric packing elements establishing sealing integrity in the annulus between inner and outer conduits in the well. Conventional packing elements are fabricated from elastomers, such as nitrile. Unfortunately, these conventional elastomers cannot withstand the hostile conditions, such as elevated temperatures and pressures, which can be encountered in some subterranean well environments. For example, in some circumstances it may be necessary to inject saturated steam into a well in order to reduce the viscosity of the crude to facilitate production of the formation fluids. Thermal well tools, such as thermal packers or steam packers, have been employed in such completions to seal the annulus between the production tubing and the casing when steam is injected through the production tubing. Unfortunately, the performance of thermal and steam packers in the presence of the saturated steam injected into the formation has proved to be less than satisfactory. The packing elements used on thermal packers have tended to have a greater leakage rate than desired, and the conventional steam packings employed on thermal packers have tended to deteriorate at an undesirable rate.
A common conventional steam packing used for thermal and steam packers consists of jacketed bulk carbonite packings consisting of fibrous asbestos impregnated with flaked graphite or mica. Asbestos packing elements reenforced with high strength nickel, copper (Monel) wire has also been used as high temperature packing element. These reenforced packing elements have also been impregnated with suitable sealing compounds to increase their usefulness at high temperatures.
In addition to fibrous asbestos seals, thermoplastic seals, such as polytetrafluoroethylene seals, have been used to increase the maximum effective temperature at which sealing integrity can be maintained. Wire mesh extrusion elements have been used on opposite ends of the primary thermoplastic sealing elements to prevent axial extrusion under hostile temperature and pressure conditions. These intermediate hard thermoplastics, such as polytetrafluoroethylene (Teflon), while more resilient than fibrous asbestos, do not possess the resiliency of conventional elastomers, such as nitrile. Attempts have been made to improve the performance of these thermoplastic seals by impregnating asbestos fibers with polytetrafluoroethylene interwoven with a nickel base chromium iron alloy (Inconel) wire. The addition of the asbestos fibers and Inconel wire to form a composite thermoplastic packing element has been intended to improve the resiliency of the thermoplastic members at high temperature. However, there does exist an upper limit at which thermoplastic sealing elements become too plastic to retain significant ability to establish sealing integrity.
The use of nonpolymeric, nonelastomeric seals in geothermal applications under conditions of high temperature and pressure has also been investigated. Laminated graphite seals bonded to metallic rings, such as berilium copper rings to impart resiliency to the graphite, have been tested for use as rotary seals in geothermal applications. Chevron shaped seals formed from powdered graphite cold pressed into a woven metal core have also been tested for use in the same applications. In each case, metallic elements have been added to the graphite sealing structures to impart resiliency to the seals. These composite structures are formed by bonding or intertwining metal members to graphite laminate ribbons formed by cold pressing expanded graphite particles to maintain a compression set in the form of substantially flat flexible integrated graphite sheets or ribbons. The graphite itself is substantially nonresilient and therefore comprises a nonenergizing seal.
In addition to employing high temperature packing elements to establish integrity between the production tubing and the casing of a well, thermal packers also employ dynamic seals acting between inner and outer mutually reciprocal tubular members comprising an integral expansion joint. Integral expansion or slick joints are commonly included in high temperature well tools, such as thermal or steam packers, to account for the expansion and contraction of the production tubing as a result of pressure fluctuations. Normally, thermal or steam packers are first anchored in place and the annular thermal packing elements are radially expanded to establish sealing integrity between the production tubing and the casing. The integral expansion or slick joint is then released relative to the stationary packer. Adequate travel is provided for the integral expansion joint to account for anticipated expansion under high temperature conditions.